1. Field of the Invention
This invention relates to RF systems and more particular to directional couplers.
2. Description of the Related Art
The physical implementation of radio frequency (RF) information transmission systems requires the amplification of the RF signal before it is injected into the communication channel. Examples of communication channels are free space in conjunction with antennas, coaxial cables, and wave guides. The amplification of the RF signal is provided by the RF power amplifier (PA). Many systems that include an RF power amplifier require sensing the amount of power generated at the output port of the PA for multiple reasons, e.g., power level control by way of feedback, reliability, and safety. A common approach to satisfying the requirement specified above is the addition of a directional coupler inserted between the RF PA output and the point of injection of the RF signal into the communication channel. The directional coupler siphons a very small but predictable portion of the RF signal destined for the communication channel and presents it to a dedicated port, the coupled port, where it is evaluated by other subsystems present in the system.
A directional coupler is an RF component typically comprising four signal ports: an input port, an output port, a coupled port, and an isolated port. FIG. 1 shows a portion of an RF system according to prior art. The output 9 of the RF power amplifier block 10 is coupled directly to the input port of the directional coupler. The output port of the directional coupler, node 3, is coupled to the input of the communication channel illustrated in FIG. 1 as resistor 6. The communication channel input impedance is usually the characteristic impedance of an RF transmission line, customarily 50-ohm. The directional coupler of FIG. 1 has the coupled port shown as node 4 and the isolated port shown as node 5. The isolated port, node 5, is connected to resistor 7, typically 50-ohm, illustrating either a physical resistor or in general terms, any port that presents an impedance appropriate for the described connection. When the PA injects RF signal power on node 9, a fraction, defined as the coupling ratio, of the RF power traveling towards the load 6, typically −20 dB (1%), appears at the coupled port 4. The directional coupler is designed such that the coupled port 4 presents a signal substantially representative of the RF power traveling from the PA 10 towards the load 6, while the isolated port 5 presents a signal substantially representative of the RF power traveling from the load 6 towards the PA 10. A significant figure of merit for the directional coupler is represented by the directivity of the coupler defined as the ratio of the power presented at the coupled port to the power presented at the isolated port, in the presence of a perfect impedance match at node 3.
As further shown in FIG. 1, the PA 10 includes an RF amplifier stage 1 and a matching network 2. The matching network 2 plays the role of an impedance transformation network, which converts the relatively high load impedance (e.g., 50-ohm) into a lower impedance (e.g., 5-ohm) as seen by the output of the final RF amplifier stage 1. It is common practice in the art to design the matching network to expect a load impedance on node 9 of 50-ohm. It is also common practice in the art to design the directional coupler 8 to expect a load impedance on node 3 of 50-ohm and further present an impedance of 50-ohm at node 9 so as to satisfy the 50-ohm expectation of the matching network 2. Moreover, the directional coupler is designed for 50-ohm impedances at the coupled port 4 and isolated port 5.
Various prior art embodiments have demonstrated the ability to construct the three parts shown in FIG. 1 within a PA module that satisfies the requirements set forth above. However, both the best achievable module performance and the smallest achievable physical dimensions of the module implementation are hampered by the rigid system partition depicted in FIG. 1.